1. Field of the Invention
The present invention relates to a process for producing a resilient roll having a hard roll core (such as a metal) and a resilient covering layer. In forming the covering layer, an inner connecting layer is first applied to the roll core and then an outer functional layer is applied to the connecting layer. Furthermore, the invention is directed towards a roll, in particular for smoothing paper webs, having a hard roll core (such as a metal), which is provided on its outer periphery with a resilient covering layer. The resilient covering layer can comprise a radially outer functional layer and a radially inner connecting layer for connecting the functional layer to the roll core.
2. Discussion of Background Information
Resilient rolls of this type are used, for example, in the calendering of paper webs. An elastic roll and a hard roll form a press nip, through which the paper web to be processed is directed. While the hard roll has a very smooth surface, consisting, for example, of steel or hard cast iron, and is responsible for smoothing that side of the paper web which faces it, the resilient (i.e., second) roll, acting on the opposite side of the paper web serves to compact the paper web and then renders it uniform in the press nip. The resilience of the second roll therefore prevents a too intensive compaction of the paper web, which would lead to a specky appearance of the paper web.
The rolls are of the order of from about 6 to 12 m long and of from about 800 to 1500 mm in diameter. The rolls withstand line forces of up to approximately 600 N/mm and compressive stresses of up to approximately 50 N/mm.sup.2. Since the tendency or trend in paper manufacture is for calendering to be carried out on-line (i.e., the paper web leaving the papermaking machine or coating machine is immediately led through the paper smoothing or calendering device), higher requirements than previously are placed on the rolls of the smoothing device, particularly with respect to their temperature resistance. The high transport speeds of the paper web, necessitated by on-line operation, and the associated high rotational speeds of the calender rolls, increase their alternating flexure frequency, which in turn leads to increased roll temperatures. These high temperatures, produced in on-line operation, lead to problems which, in the case of known resilient rolls, can lead to the destruction of the synthetic covering of the roll. On the one hand, in the case of known synthetic coverings, maximum temperature differences of about 20.degree. C. over the width of the roll are permissible. On the other hand, the polymers normally used for the coating have a significantly higher coefficient of thermal expansion than the steel or hard cast iron rolls normally used. Accordingly, as a result of an increase in the temperature, high axial stresses occur between the steel or hard cast iron roll and the synthetic coating connected to it.
As a result of these high stresses, associated with heated locations, occurring at certain points or regions within the synthetic coating, so-called hot spots can occur, at which the separation or even the bursting of the synthetic layer takes place.
These hot spots tend to occur in particular when, in addition to the mechanical stresses and the relatively high temperature, there are crystallization points in the form of, for example, faulty adhesive bonds, deposits or above-average bulges in the resilient covering. These bulges can occur, for example, as a result of creases or foreign bodies on the paper web. In such cases, the temperature at these crystallization points can rise from a normal of about 80.degree. C. to 90.degree. C. to more than 150.degree. C., which results in the aforementioned destruction of the synthetic layer.